1. Field of the Invention
The present invention disclosed herein relates to image sensors and more particularly, to a complementary metal oxide semiconductor (CMOS) image sensors which compare a sensing signal with a reference signal.
2. Description of the Related Art
CMOS image sensors widely used in digital cameras convert optical signals into electrical signals. This conversion occurs in pixels of the CMOS image sensor, each of which includes a photodiode and a read-out circuit. The photodiode generates charges induced by light that it absorbs. The charges generated are converted into an analog voltage and transferred to the read-out circuit. The read-out circuit converts the analog voltage into digital data.
An analog-to-digital conversion is performed by comparing the analog voltage of a pixel to a ramp voltage by means of a comparator. A counting value when the ramp voltage reaches to the analog voltage is adopted as the digital data converted from the analog voltage.
Generally, the pixel is structured to include four transistors. A first NMOS transistor initializes the pixel. A second NMOS transistor transfers the pixel's image information. A third NMOS transistor functions to select the pixel. A fourth NMOS transistor forms a source follower as a buffer for transferring the image information of the pixel.
However, when the pixel transfers a sensing signal (floating diffusion) to the read-out circuit through the source follower, the response signal is limited in a dynamic range and noise is increased.